Vehicle windshield wipers must provide some means to maintain the wiper blade pressurized against the windshield as it sweeps back and forth. Most commonly, this is done with a tension spring stretched between a base fixed to a drive shaft and a wiper arm pivoted to the base The pivot axis is above the line of force of the spring, so a moment arm is established and continual torque is applied to the wiper arm about the pivot axis that maintains pressure on a wiper blade mounted to the arm. Since it is not desirable that the wiper arm sit very high off of the windshield, the moment arm through which the spring force acts cannot be long. However, if the spring is strong enough, the torque it creates is great enough to maintain the wiping pressure, at least under most circumstances.
One circumstance where the wiping pressure may become inadequate, at least with springs of conventional strength, is at high vehicle speeds. Above a certain vehicle speed, wind lift on the blade and arm may counteract the spring sufficiently to lessen the wiping pressure. When combined with high winds in the opposite direction, the net wind acting on the blade may have an even greater effect. Providing a spring strong enough to stand up to the worst case potential wind lift would create a wiping pressure that was too high the rest of the time. Consequently, many designs for wipers with adjustable wiping force have been proposed. A common feature of most is a motor driven actuator that changes the angle of force of the tension spring to change the torque and wiping pressure that it applies Such designs also are generally bulky, complex and expensive.